There has been a recent paradigm shift in the intensive care unit (ICU) setting toward limiting blood transfusion. This has been less the consequence of demonstration that such products are not needed by patients than by recognition of the myriad of negative effects triggered or potentiated by transfusion. Similarly, synthetic blood products have fallen short of therapeutic expectations. Contrary to expectation, transfusion of stored red blood cells (RBCs) has not been shown to improve tissue oxygenation. Many of the detrimental effects are related to the loss of the RBC ability to mediate nitric oxide (NO)-centered vasodilatory activity. Critical interactions occur between S-Nitrosothiols (SNO), Hemoglobin (Hb) and the RBC membrane which allow the endogenous RBC to act as a local oxygen sensor and a hypoxia-responsive transducer of NO signals. Recent (unpublished) work has confirmed that SNO-Hb levels decline by 70% after one day of storage and by 83% after one week of storage. This is likely to contribute strongly to the previously demonstrated severe impairment of vasodilatory activity of stored blood. We will use animal models of transfusion and aortic ring bioassays to investigate the role of nitric oxide in RBC transfusion. We will explore the hypotheses: (1) depletion of nitric oxide in transfused RBCs is detrimental to hypoxic vasodilation, (2) transfusion of RBCs into a nitric oxide deficient environment hinders the ability of RBCs to improve tissue oxygenation, (3) repletion of NO in stored RBCs with or without the coinfusion of a nitric oxide donor will improve tissue oxygenation, (4) the ability of nitric oxide repleted RBCs to improve tissue oxygenation will be dependent on the endogenous production of nitric oxide and will be time-limited in shock environments. Development of a detailed understanding of the role of NO in the activity of transfused RBCs will prove critical to the optimal management of anemic ICU patients as well as patients suffering hemorrhagic shock. The aforementioned paradigm shift occurred, not because blood products or synthetic products are not needed for management, but because the storage, development and administration of such products still need to be improved toward a balance wherein the benefits more consistently outweigh the risks. Delineating the importance and function of NO in transfusion is pivotal to this process. It is expected to advance both conventional transfusion practice as well as successful production of synthetic blood products.